1. Field of the Invention
The present invention relates to a method for continuously and simultaneously measuring an impedance and a biopotential signal on a biological subject.
The invention also relates to an electronic medical device for continuously and simultaneously measuring an impedance and a biopotential signal on a biological subject.
The invention also relates to the use of such an electronic medical device for measuring a biopotential signal (Electrocardiogram (ECG), Electroencephalogram (EEG), Electromyogram (EMG), etc.).
2. Description of the Related Technology
Several readout circuits for ambulatory monitoring of biopotential signals have been proposed in the prior art. For battery powered portable devices that are continuously monitoring electronic signals, power efficiency is of primary importance to guarantee sufficient autonomy.
In addition, field tests have revealed that motion artifacts are a significant problem for accurate and robust signal acquisition, in order to differentiate between biological information and unwanted motion artifacts. Movement artifacts are the biggest source of noise in mobile ECG recordings. These artifacts are potentials that are superimposed onto the ECG signal. These potentials occur in the electrode cables, in the skin and at the electrode/electrolyte interface. While artifacts coming from cables can be reduced by appropriate electrode cables, artifacts from the skin and electrode/electrolyte interface are difficult to reduce by design.
R. F. Yazicioglu et al. have described in “Ultra-Low-Power Wearable Biopotential Sensor Nodes,” IEEE EMBS conf., September 2009, a circuit (see FIG. 2) that injects an AC current and measures the voltage generated across the electrode impedance to continuously monitor the electrode impedance. The AC current frequency is selected to be large enough compared to the biopotential signals so that selective filtering at the backend can be used to differentiate between the biopotential signal and the voltage generated due to the electrode impedance. A disadvantage of the disclosed approach is that it requires the use of sinusoidal current sources and analog multipliers and also it requires a high sampling rate in the subsequent analog to digital conversion step (ADC), where both leads to high power consumption.
US 2008/0183098 A1 describes a chopper stabilized instrumentation amplifier for measuring different types of physiological information captured by means of an implantable pulse generator implanted in a patient.